"Stay yet awhile! speak to me once again; Kiss me, so long but as a kiss may live; And in my heartless breast and burning brain That word, that kiss, shall all thoughts else survive, With food of saddest memory kept alive, Now thou art dead, as if it were a part Of thee, my Adonais! I would give All that I am to be as thou now art! But I am chain'd to Time, and cannot thence depart!
She begs Adonais to stay, but he can't.
So she begs him to kiss her, figuring that at least his kiss could live on her lips after his body has dies. It's kind of like keeping a part of him with her.
Urania also mentions her own immortality here (remember, she's a goddess). She's sad that she can't go with him, "chain'd to Time" (immortal) as she is.
She also says that she'd give anything to be dead like he is, so that she could be with him, but, alas, she's not human. She doesn't even have a heart.
With this, Shelly is pointing out how all our efforts to save Keats are futile. It's just too late.